This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-119423 filed on Apr. 20, 2000.
1. Field of the Invention
The present invention relates to a cooling system for cooling an interior of a substantially airtight housing, such as a housing of a base station of a cellular phone system.
2. Description of Related Art
One type of cooling system for cooling an interior of a substantially airtight housing exchanges heat between internal air located within the housing and external air located outside of the housing to cool the interior of the housing.
This type of cooling system is used, for example, in a base station of a cellular phone system. When a temperature of the external air gets cold (e.g., below freezing temperature), performance of electronic devices arranged, for example, in a transmitter-receiver of the base station is degraded. To address this problem, such a cooling system generally includes an electric heater that heats the internal air located within the housing of the base station.
A power supply circuit that supplies power to the electric heater includes thermostats to prevent abnormally high heat generation of the electric heater. Furthermore, a thermal fuse is additionally provided in the power supply circuit to provide redundancy in case of failure of the thermostats.
The U.S. safety standard requires to provide the thermostat on both sides of the electric heater in the power supply circuit that supplies power to the electric heater. This requirement poses a limitation on arrangement of the thermostats. That is, in case of providing only one thermostat, the best location of the thermostat (such as one location where temperature is close to that of the electric heater and where the thermostat can be easily placed) can be easily selected. However, if two thermostats need to be provided, it is difficult to place both the thermostats at the best locations within a limited space. Furthermore, in order to prevent opening of the thermal fuse before opening of the thermostats, positional relationship between the two thermostats and the thermal fuse needs to be concerned.
The present invention addresses the described disadvantages. Thus, it is an objective of the present invention to provide a cooling system having a heater unit that includes a couple of thermostats arranged in a manner that achieves a higher degree of safety.
To achieve the objective of the present invention, there is provided a cooling system for cooling an interior of a substantially airtight housing. The cooling system includes an internal-air passage, an external-air passage, an internal-air fan, an external-air fan, a heat exchanger and a heater unit. The internal-air passage has an internal-air inlet and an internal-air outlet. Both the internal-air inlet and the internal-air outlet communicate with an interior of the housing. The external-air passage has an external-air inlet and an external-air outlet. Both the external-air inlet and the external-air outlet communicate with an exterior of the housing. The internal-air fan takes in internal air from the interior of the housing through the internal-air inlet and expels the internal air into the interior of the housing through the internal-air outlet. The external-air fan takes in external air from the exterior of the housing through the external-air inlet and expels the external air to the exterior of the housing through the external-air outlet. The heat exchanger exchanges heat between the internal air introduced into the internal-air passage and the external air introduced into the external-air passage. The heater unit heats the internal air introduced into the internal-air passage. The heater unit includes an electric heater, a power supply circuit, a heater protective device and a heat receiving plate. The electric heater generates heat upon energization. The power supply circuit supplies power to the electric heater. The heater protective device is interposed in the power supply circuit. The heater protective device includes a first thermostat and a second thermostat. The first thermostat is interposed in the power supply circuit on a first side of the electric heater. The second thermostat is interposed in the power supply circuit on a second side of the electric heater that is opposite to the first side of the electric heater. The first thermostat and the second thermostat are arranged to open substantially at a first predetermined temperature or higher to disconnect the power supply circuit. The heat receiving plate receives heat from the electric heater and conducts the heat of the electric heater to the heater protective device. The first thermostat and the second thermostat are secured to the heat receiving plate.